It is known about the existence on the market of a large number of coatings for external applications on a woody support. In particular in recent years, in order to reduce the emission of volatile organic substances into the environment, a series of water-based products has been developed. A group of water-based coatings for applications on wood consists of transparent coatings for external use.
It is known that the woody support and the coating undergo degradation phenomena due to atmospheric agents such as for instance solar radiation, in particular the ultraviolet component of solar radiation; heat; environment temperature; liquid humidity (rain) or gas humidity (atmospheric humidity). Moreover, it is known that a woody support exposed to sunlight undergoes a degradation process. In practice, the exposed surface takes on a gray color due to lignin degradation, which results in a higher wood porosity, a loss of structural firmness and a higher absorption of environment humidity. Such degradation involves an increase of phenomena of dimensional variation, which can be seen as surface splits. Splits or clefts make water absorption easier, which eventually results in a dark brown color and degrades little by little wood internal structure.
The increase of relative humidity favors the formation of colonies of moulds or fungi. The degradation of the woody support and of the coating, for instance a transparent coating, involves decay in the external appearance of the coated manufactured item. Once the shielding action of the coating film is over, the degradation speed of the woody support gets higher. The practical result of this degradation is an unpleasant appearance of the coated manufactured item.
In addition, a structural decay of wood takes place, due to a subsequent splitting of the wood surface. Wood then rots because of fungi and moulds, which results in a loss of dimensional stability of the material used to build for instance an external window.
Radiation curing is an attractive option for quality wood finishers. Not only does this technology assure compliance with volatile organic compounds (VOCs) and hazardous air pollutants (HAPS) environmental mandates, it is one of the most cost-effective methods for creating a premium finish that will leave a lasting impression on customers. Wooden materials can be protected from adverse factors such as visible light, UV-light, oxygen, heat, humidity and water, biological attack and air pollutants, using various protective and decorative finishes such as paints, transparent stains and penetrating finishes or film forming clear varnishes. Transparent systems that allow the natural features of the wood (color and texture) to remain visible are attracting interest and the demand for them has been increasing.
Wood is widely use as a natural raw material in construction, furniture industry and for parquetry end uses. Its physical properties and its warm appearance distinguish it in many areas from competitive materials such as concrete, metals and plastics. Coatings (paints, varnishes and lacquers) are an essential part of the majority of products based on wood-based materials. Consumer and industrial desire for more environmentally benign coatings is growing rapidly. The conventional use of acrylate ester in formulation gave low scratch resistance for wooden surfaces.
Furthermore, acrylate ester is considered a strong eye and skin irritant. Thus, industry continues to look for ways to provide coating compositions that provide better performance yet environmental friendly in the workplace.
At present, only formulations of radiation coating containing oligomers and monomers from acrylate ester have been studied (Kumar et al., 2006; Bajpai et al., 2002). However, the acrylate ester is hazardous and toxic. Therefore the need to study new radiation coating formulation from adipate ester is very important due to the fact of green technology production.
The objective of the present invention provides a formulation for a coating material wherein the coating material includes wax esters as ingredients in coating for wooden surfaces with minimum pollutants. This invention also offers a comprehensive approach to this new technology, designed to ensure that the right application components and coating systems are selected for higher throughput, lower cycle time and maximum UV efficiency. The formulation for the coating material on wooden surfaces is preferably solvent free and consists of wax ester (adipate ester and palm-based wax esters), epoxy acrylate, surfactant, hardener and photoinitiator. In general, wax esters make attractive starting materials for wood coatings, because they are safer, healthier and pleasant to use. Wax esters was tested in the industrial wood coating, which may produce and improve surfaces resistant to water, less to fat stains, highlighting the natural character of wood, long lasting and hardwearing.
Adipate esters are widely used due to their relatively low cost and good balance of properties. Simple dialkyl adipates are made by reacting adipic acid or its dimethyl ester with monohydric alcohols (Gryglewicz, 2001). Suitable fatty acids for the usage in preparing fatty esters are palm oil, palm kernel oil, palm olein and palm stearin. The uses of wax esters are attractive since there are non-hazardous compounds with good biodegradability. Most of today's commercial enzymatic processes have a variety of positive characteristics, such as high productivity and a lack of undesirable by-product. There is a strong attention in developing waxes to serve as ingredients in coatings for wooden surfaces, with minimum pollutants and with substrates from renewable resources. The conventional extractions of wax esters from plant materials and direct biosynthesis by fermentation are the two methods for organic esters synthesis.
However, these methods exhibit a high cost of processing and low yields of desired esters and therefore, better processes need to be developed and to cater the environmental benign processes. Chemical routes have to attend problems such as poor reaction selectivity and extreme reaction condition leading to undesirable side reactions, low yields, pollution and high cost of manufacturing. As an alternative, the use of lipases to catalyze these synthesis reactions has recently become a much more promising method (Rejassa, et al., 2003). Lipase-catalyzed reactions are superior to conventional chemical methods owing to mild reaction conditions, high catalytic efficiency and the inherent selectivity of the natural catalysts which results in much purer products. In recent years, enzymatic catalyzed reaction has been widely understood and able to produce a highly pure product at mild temperature and atmospheric pressure (Abdul Rahman et al., 2003).